It is known to remove oxides of nitrogen (NO.sub.x) pollutants from exhaust streams or other gaseous emission sources which contain excess oxygen by adding NH.sub.3 to the gas stream in a proportion based on the amount of NO.sub.x present and reacting the NH.sub.3 and NO.sub.x over a catalyst to form non-polluting N.sub.2 and H.sub.2 O.
A wide variety of catalysts have been proposed for this purpose. See, for example, U.S. Pat. Nos. 3,279,884 and 4,115,516. Vanadium pentoxide or other oxide supported on alumina has been extensively used as an SCR catalyst. Cerium oxide has also been proposed. See, for instance, the abovementioned U.S. Pat. No. 4,115,516 as well as U.S. Pat. No. 3,885,019.
Oxysulfur compounds of manganese, iron and nickel have also been disclosed as active SCR catalysts in U.S. Pat. No. 4,107,272. However, it has been found that manganese and nickel catalysts prepared according to the prior art have low SCR activities and cannot, therefore, be used commercially. Iron sulfate catalysts prepared according to the prior art do provide commercially useful SCR performance over short periods of time but they are not durable and their activity diminishes in use, particularly when they are employed in the processing of flue gases containing only low concentrations of sulfur oxides (SO.sub.x).
The principal object of the present invention is to provide SCR catalysts based on the use of one or more oxysulfur compounds of manganese and/or nickel, and optionally iron oxysulfur compounds, which demonstrate a commercially useful SCR activity that is durable over long periods of use whether or not sulfur oxides are present.